Image display device

ABSTRACT

An image display device of the present disclosure includes an image light generating device, a first, a second, a third, and a fourth optical unit. A first intermediate image is formed between the first and the third optical unit. A pupil is formed between the second and the fourth optical unit. A second intermediate image is formed between the third and the fourth optical unit. An exit pupil is formed at an opposite side of the fourth optical unit from the third optical unit. The image light generating device includes a first, a second, a third light emitting panel, and a color synthesis element. The color synthesis element is constituted of a cross dichroic prism including a first and a second dichroic film that intersect with each other. Each of the first and the second dichroic film does not have a polarization separation characteristic.

The present application is based on, and claims priority from JPApplication Serial Number 2018-210560, filed Nov. 8, 2018, thedisclosure of which is hereby incorporated by reference herein in itsentirety.

BACKGROUND 1. Technical Field

The present disclosure relates to an image display device.

2. Related Art

A head-mounted display device configured to guide image light to an eyeof an observer while reflecting the image light by using a plurality ofreflective surfaces is known. JP-A-2012-18414 discloses a head-mounteddisplay device including a frame, an image generating device, alight-guiding plate that guides light emitted from the image generatingdevice to an eye of an observer, and a first deflection means and asecond deflection means for reflecting the light incident on thelight-guiding plate.

However, in the head-mounted display device in JP-A-2012-18414, thelight-guiding plate is used as a means for guiding light emitted fromthe image generating device to an eye of an observer. For this reason,there is a problem in that the device becomes large and heavy. Thus,JP-A-2017-167181 discloses a display device configured to guide imagelight to an eye of an observer by using two diffraction elementsconstituted of a reflection-type hologram. In JP-A-2017-167181, acombination of a laser light source and a scanning optical system, aliquid crystal panel, an organic electroluminescence (EL) panel, and thelike are exemplified as an image light generating device.

It can be said that a reflection-type hologram described inJP-A-2017-167181 is an optical system suitable for a see-through imagedisplay device configured to superimpose external light on a display fordisplay since the reflection-type hologram reflects only light in aspecific wavelength region and transmits light in the other wavelengthregion. However, a reflective wavelength width is generally very narrowin a reflection-type hologram, and thus a lot of image light emittedfrom a display is not reflected by the hologram and is transmittedthrough the hologram. Thus, there is a problem in that light-guidingefficiency is low.

It is conceivable to use, as an image light generation device, anorganic EL panel that can achieve higher contrast than that of a liquidcrystal panel, for example. The organic EL panel has features such aslow profile and light weight, and is expected to be applied to a directview display, of course, and also a head-mounted display device incombination with the aforementioned reflection-type hologram. However,when the organic EL panel is combined with an optical system having lowlight-guiding efficiency, such as the reflection-type hologram, there isa problem in that, when panel luminance is increased, a life of anorganic EL element is reduced, and brightness deteriorates quickly.

SUMMARY

To solve the above-described problem, an image display device accordingto one aspect of the present disclosure includes an image lightgenerating device, a first optical unit having positive power, a secondoptical unit including a first diffraction element and having positivepower, a third optical unit having positive power, and a fourth opticalunit including a second diffraction element and having positive power,the first to fourth optical units being provided along an optical pathof image light emitted from the image light generating device. On theoptical path, a first intermediate image of the image light is formedbetween the first optical unit and the third optical unit, a pupil isformed between the second optical unit and the fourth optical unit, asecond intermediate image of the image light is formed between the thirdoptical unit and the fourth optical unit, and an exit pupil is formed atan opposite side of the fourth optical unit from the third optical unit.The image light generating device includes a first light emitting panelconfigured to emit first image light in a red wavelength region, asecond light emitting panel configured to emit second image light in agreen wavelength region, a third light emitting panel configured to emitthird image light in a blue wavelength region, and a color synthesiselement configured to synthesize the first image light, the second imagelight, and the third image light. The color synthesis element isconstituted of a cross dichroic prism including a first dichroic filmand a second dichroic film that intersect with each other, and each ofthe first dichroic film and the second dichroic film does not have apolarization separation characteristic.

In the image display device according to one aspect of the presentdisclosure, each of the first light emitting panel, the second lightemitting panel, and the third light emitting panel may face a lightincident surface of the cross dichroic prism, and may be disposed suchthat a longitudinal direction of an image generation region is parallelto a cross axis of the first dichroic film and the second dichroic film.

In the image display device according to one aspect of the presentdisclosure, each of the first light emitting panel, the secondself-light-emitting panel, and the third light emitting panel mayinclude a pixel including an organic EL element.

In the image display device according to one aspect of the presentdisclosure, the organic EL element may include an optical resonator.

In the image display device according to one aspect of the presentdisclosure, each of the first light emitting panel, the second lightemitting panel, and the third light emitting panel may include a pixelincluding an inorganic light-emitting diode element.

In the image display device according to one aspect of the presentdisclosure, the first diffraction element and the second diffractionelement each may be constituted of a reflection-type volume hologram.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external view illustrating one aspect of a display deviceaccording to a first exemplary embodiment.

FIG. 2 is an external view illustrating another aspect of the displaydevice.

FIG. 3 is a schematic diagram illustrating one aspect of an opticalsystem of the display device.

FIG. 4A is a schematic diagram of interference fringes of a diffractionelement.

FIG. 4B is a schematic diagram illustrating another aspect of theinterference fringes of the diffraction element.

FIG. 5 is a diagram illustrating a diffraction characteristic of a firstdiffraction element and a second diffraction element.

FIG. 6A is a schematic diagram of a case in which the first diffractionelement and the second diffraction element are in a conjugatedrelationship.

FIG. 6B is a schematic diagram of a case in which the first diffractionelement and the second diffraction element are not in the conjugatedrelationship.

FIG. 6C is a schematic diagram of a case in which the first diffractionelement and the second diffraction element are not in the conjugatedrelationship.

FIG. 7A is a schematic diagram illustrating a tolerance for a deviationfrom the conjugated relationship between the first diffraction elementand the second diffraction element.

FIG. 7B is a schematic diagram illustrating another aspect of thetolerance for the deviation from the conjugated relationship between thefirst diffraction element and the second diffraction element.

FIG. 8 is a light beam diagram of the optical system.

FIG. 9 is a perspective view of an image light generating device.

FIG. 10 is a schematic cross-sectional view illustrating a configurationof one pixel included in a light emitting panel.

FIG. 11 is a diagram illustrating one example of atransmittance-wavelength characteristic of a dichroic film that does nothave a polarization separation characteristic.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the present disclosure will be described belowwith reference to the accompanying drawings.

In each of the figures below, to illustrate each of layers or each ofmembers at a recognizable size, a scale of each of the layers or each ofthe members is different from an actual scale and an actual angle.

FIG. 1 is an external view illustrating one aspect of an image displaydevice 100 according to the present exemplary embodiment. FIG. 2 is anexternal view illustrating another aspect of the image display device100. FIG. 3 is a schematic diagram illustrating one aspect of an opticalsystem 10 of the image display device 100 illustrated in FIG. 1.

In FIGS. 1 to 3, a front-rear direction is a direction along a Z axis, afront direction being one of the front-rear direction is a front sideZ1, and a back direction being the other front-rear direction is a rearside Z2. Further, a left-and-right direction is a direction along an Xaxis, a right direction being one of the left-and-right direction is aright side X1, and a left direction being the other left-and-rightdirection is a left side X2. Further, an up-and-down direction is adirection along a Y-axis direction, an upper direction being one of theup-and-down direction is an upper side Y1, and a lower direction beingthe other up-and-down direction is a lower side Y2.

As illustrated in FIG. 1, the image display device 100 is a head-mounteddisplay device, and includes a right-eye optical system 10 a that causesimage light L0 a to be incident on a right eye Ea and a left-eye opticalsystem 10 b that causes image light L0 b to be incident on a left eyeEb. For example, the image display device 100 is formed in a shape likeglasses.

Specifically, the image display device 100 includes a frame 90 thatholds the right-eye optical system 10 a and the left-eye optical system10 b. The frame 90 is mounted on a head of an observer. The frame 90 hasa front portion 91 that holds a second diffraction element 70 a of theright-eye optical system 10 a and a second diffraction element 70 b ofthe left-eye optical system 10 b. A temple 92 a on a right side of theframe 90 and a temple 92 b on a left side respectively hold an imagelight projecting device of the right-eye optical system 10 a and animage light projecting device of the left-eye optical system 10 b.

The right-eye optical system 10 a and the left-eye optical system 10 bhave the same basic configuration. Therefore, the right-eye opticalsystem 10 a and the left-eye optical system 10 b will be collectivelydescribed as the optical system 10 without distinction in thedescription below.

In the image display device 100 illustrated in FIG. 1, the image lightL0 travels in the left-and-right direction along the X axis. However, asillustrated in FIG. 2, a configuration in which the image light L0travels from the upper side Y1 to the lower side Y2 and is emitted to aneye E of an observer, a configuration in which the optical system 10 isdisposed from a head top portion to the front of the eye E, and the likemay be applied.

A basic configuration of the optical system 10 of the image displaydevice 100 will be described with reference to FIG. 3.

FIG. 3 is a schematic diagram illustrating one aspect of the opticalsystem 10 of the image display device 100 illustrated in FIG. 1. Notethat FIG. 3 illustrates, in addition to light L1 (solid line) having aspecific wavelength of the image light L0, light L2 (dot-and-dash line)on a long wavelength side with respect to the specific wavelength, andlight L3 (dashed line) on a short wavelength side with respect to thespecific wavelength.

As illustrated in FIG. 3, in the optical system 10, a first optical unitL10 having positive power, a second optical unit L20 having positivepower, a third optical unit L30 having positive power, and a fourthoptical unit L40 having positive power are disposed along a travelingdirection of the image light L0 emitted from an image light generatingdevice 31.

In the present exemplary embodiment, the first optical unit L10 havingpositive power is constituted of a projection optical system 32. Thesecond optical unit L20 having positive power is constituted of a firstdiffraction element 50 of a reflection type. The third optical unit L30having positive power is constituted of a light-guiding optical system60. The fourth optical unit L40 having positive power is constituted ofa second diffraction element 70 of a reflection type. In the presentexemplary embodiment, the first diffraction element 50 and the seconddiffraction element 70 are constituted of reflection-type volumeholograms 85 and 86 that are described later.

In the optical system 10, with a focus on the traveling direction of theimage light L0, the image light generating device 31 emits the imagelight L0 toward the projection optical system 32, the projection opticalsystem 32 emits the incident image light L0 toward the first diffractionelement 50, and the first diffraction element 50 emits the incidentimage light L0 toward the light-guiding optical system 60. Thelight-guiding optical system 60 emits the incident image light L0 towardthe second diffraction element 70, and the second diffraction element 70emits the incident image light L0 toward the eye E of the observer.

The image light generating device 31 generates the image light L0. Adetailed configuration of the image light generating device 31 will bedescribed later.

The projection optical system 32 projects the image light L0 generatedby the image light generating device 31. The projection optical system32 includes a plurality of lenses 321. In FIG. 3, an example is given ofa case in which the projection optical system 32 includes three lenses321. However, the number of lenses 321 is not limited, and theprojection optical system 32 may include four or more lenses 321.Further, the projection optical system 32 may be constituted in a formin which the plurality of lenses 321 are bonded together. Further, thelens 321 may be constituted of a free-form lens.

The light-guiding optical system 60 includes a lens system 61 on whichthe image light L0 emitted from the first diffraction element 50 isincident and a mirror 62 that emits the image light L0 emitted from thelens system 61 in a direction inclined diagonally. The lens system 61 isconstituted of a plurality of lenses 611 arranged in the front-reardirection along the Z axis. The mirror 62 includes a reflective surface620 inclined diagonally to the front-rear direction. In the presentexemplary embodiment, the mirror 62 is constituted of a total reflectionmirror. However, the mirror 62 may be a half mirror, and in this case, arange in which the external light is visually recognizable can bewidened.

A detailed configuration of the first diffraction element 50 and thesecond diffraction element 70 will be described below.

In the present exemplary embodiment, the first diffraction element 50and the second diffraction element 70 have the same basic configuration.Hereinafter, a configuration of the second diffraction element 70 willbe described as an example.

FIG. 4A is a schematic diagram of interference fringes 751 of the seconddiffraction element 70 illustrated in FIG. 3.

As illustrated in FIG. 4A, the second diffraction element 70 includesthe reflection-type volume hologram 85. The reflection-type volumehologram 85 is a partially reflection diffraction optical element. Thus,the second diffraction element 70 constitutes a partial transmissivereflective combiner. Therefore, external light is also incident on theeye E via the second diffraction element 70, and thus the observer canrecognize an image in which the image light L0 formed by the image lightgenerating device 31 and the external light (background) aresuperimposed on each other.

The second diffraction element 70 faces the eye E of the observer. Theincident surface 71 of the second diffraction element 70 on which theimage light L0 is incident has a concave surface being recessed in adirection away from the eye E. In other words, the incident surface 71has a shape having a central portion recessed and curved with respect toa peripheral portion in the incident direction of the image light L0.Thus, the image light L0 can be efficiently condensed toward the eye Eof the observer.

The second diffraction element 70 includes interference fringes 751R,751G, and 751B having a pitch corresponding to a specific wavelength.The interference fringes 751R, 751G, and 751B are recorded as adifference in refractive index and the like in a hologram photosensitivelayer. The interference fringes 751R, 751G, and 751B are inclined in onedirection with respect to the incident surface 71 of the seconddiffraction element 70 so as to correspond to a specific incident angle.Therefore, the second diffraction element 70 diffracts and then deflectsthe image light L0 in a predetermined direction. The specific wavelengthand the specific incident angle respectively correspond to a wavelengthand an incident angle of the image light L0. The interference fringes751R, 751G, and 751B can be formed by performing interference exposureon the holographic photosensitive layer by using reference light Lr andobject light Ls.

In the present exemplary embodiment, the image light L0 is image lightfor color display. Thus, the second diffraction element 70 includes theinterference fringes 751R, 751G, and 751B having a pitch correspondingto the specific wavelength. For example, the interference fringes 751Rare formed at a pitch corresponding to red light LR having a wavelengthof 615 nm, for example, in a wavelength range from 580 nm to 700 nm. Theinterference fringes 751G are formed at a pitch corresponding to greenlight LG having a wavelength of 535 nm, for example, in a wavelengthrange from 500 nm to 580 nm. The interference fringes 751B are formed ata pitch corresponding to blue light LB having a wavelength of 460 nm,for example, in a wavelength range from 400 nm to 500 nm. Theinterference fringes 751R, 751G, and 751B of this kind are formed byforming a holographic photosensitive layer having sensitivitycorresponding to the respective wavelengths, and performing interferenceexposure on the holographic photosensitive layer by using referencelight LrR, LrG, and LrB and object light LsR, LsG, and LsB having therespective wavelengths.

Note that a photosensitive material having sensitivity corresponding tothe respective wavelengths may be dispersed in the holographicphotosensitive layer, and then interference exposure may be performed onthe holographic photosensitive layer by using the reference light LrR,LrG, and LrB and the object light LsR, LsG, and LsB having therespective wavelengths. In this way, as illustrated in FIG. 4B, theinterference fringes 751 in which the interference fringes 751R, 751G,and 751B are superimposed on one layer may be formed. Further, lighthaving a spherical wave may be used as the reference light LrR, LrG, andLrB and the object light LsR, LsG, and LsB.

The first diffraction element 50 having the same basic configuration asthe second diffraction element 70 includes the reflection-type volumehologram 86. An incident surface 51 of the first diffraction element 50on which the image light L0 is incident has a concave surface beingrecessed. In other words, the incident surface 51 has a shape having acentral portion recessed and curved with respect to a peripheral portionin the incident direction of the image light L0. Thus, the image lightL0 can be efficiently deflected toward the light-guiding optical system60.

FIG. 5 is a schematic diagram illustrating a diffraction characteristicof the first diffraction element 50 and the second diffraction element70 illustrated in FIG. 3. FIG. 5 illustrates a difference in diffractionangle between a specific wavelength and a peripheral wavelength when alight beam is incident on one point on the volume hologram. In FIG. 5,when assuming that a specific wavelength is 531 nm, a deviation indiffraction angle of light having a peripheral wavelength of 526 nm isindicated by a solid line L526, and a deviation in diffraction angle oflight having a peripheral wavelength of 536 nm is indicated by a dashedline L536.

As illustrated in FIG. 5, even when a light beam is incident on the sameinterference fringes recorded in the hologram, the light beam having alonger wavelength is more diffracted, and the light beam having ashorter wavelength is less diffracted. Thus, when two diffractionelements, namely, the first diffraction element 50 and the seconddiffraction element 70 are used as in the present exemplary embodiment,wavelength compensation cannot be appropriately performed unless lighthaving a long wavelength and light having a short wavelength are eachincident in consideration of light beam angles of the light having along wavelength and the light having a short wavelength with respect toa specific wavelength. In other words, a color aberration generated bythe second diffraction element 70 cannot be canceled unless the light isincident in consideration of the light beam angle for each wavelength.Further, diffraction angles vary depending on the number of interferencefringes, and thus the interference fringes need to be considered.

In the optical system 10 illustrated in FIG. 3, an incident directionand the like to the second diffraction element 70 are made appropriateaccording to whether a sum of the number of times of formation of anintermediate image between the first diffraction element 50 and thesecond diffraction element 70 and the number of times of reflections bythe mirror 62 is odd or even. Thus, wavelength compensation, namely, acolor aberration can be canceled.

Specifically, as illustrated in FIG. 3, the image light L0 incident onthe first diffraction element 50 is diffracted and deflected by thefirst diffraction element 50. At this time, the light L2 on the longwavelength side with respect to the specific wavelength has adiffraction angle θ2 greater than a diffraction angle θ1 of the light L1having the specific wavelength. Further, the light L3 on the shortwavelength side with respect to the specific wavelength has adiffraction angle θ3 smaller than the diffraction angle θ1 of the lightL1 having the specific wavelength. Therefore, the image light L0 emittedfrom the first diffraction element 50 is deflected and dispersed at eachwavelength.

The image light L0 emitted from the first diffraction element 50 isincident on the second diffraction element 70 via the light-guidingoptical system 60 and is diffracted and then deflected by the seconddiffraction element 70. At this time, on the optical path from the firstdiffraction element 50 to the second diffraction element 70, anintermediate image is formed once, and reflection by the mirror 62 isperformed once.

Therefore, when assuming that an angle between the image light L0 and anormal line of the incident surface of the second diffraction element 70is an incident angle, the light L2 on the long wavelength side withrespect to the specific wavelength has an incident angle θ12 greaterthan an incident angle θ11 of the light L1 having the specificwavelength while the light L3 on the short wavelength side with respectto the specific wavelength has an incident angle θ13 smaller than theincident angle θ11 of the light L1 having the specific wavelength.Further, as described above, the light L2 on the long wavelength sidewith respect to the specific wavelength has the diffraction angle θ2greater than the diffraction angle θ1 of the light L1 having thespecific wavelength. The light L3 on the short wavelength side withrespect to the specific wavelength has the diffraction angle θ3 smallerthan the diffraction angle θ1 of the light L1 having the specificwavelength.

Therefore, the light L2 on the long wavelength side with respect to thespecific wavelength is incident on the first diffraction element 50 atthe incident angle greater than the incident angle of the light L1having the specific wavelength. However, the light L2 on the longwavelength side with respect to the specific wavelength has thediffraction angle greater than the diffraction angle of the light L1having the specific wavelength, and thus, as a result, the light L2 onthe long wavelength side with respect to the specific wavelength and thelight L1 having the specific wavelength are substantially parallel lightwhen being emitted from the second diffraction element 70. In contrast,the light L3 on the short wavelength side with respect to the specificwavelength is incident on the first diffraction element 50 at theincident angle smaller than the incident angle of the light L1 havingthe specific wavelength. However, the light L3 on the short wavelengthside with respect to the specific wavelength has the diffraction anglesmaller than the diffraction angle of the light L1 having the specificwavelength, and thus, as a result, the light L3 on the short wavelengthside with respect to the specific wavelength and the light L1 having thespecific wavelength are substantially parallel light when being emittedfrom the second diffraction element 70. In this way, as illustrated inFIG. 3, since the image light L0 emitted from the second diffractionelement 70 is incident as the substantially parallel light on the eye Eof the observer, misalignment of image formation in a retina E0 at eachwavelength can be suppressed. Therefore, a color aberration generated bythe second diffraction element 70 can be canceled.

A conjugated relationship between the first diffraction element 50 andthe second diffraction element 70 will be described below.

FIG. 6A is a schematic diagram of a case in which the first diffractionelement 50 and the second diffraction element 70 are in the conjugatedrelationship. FIGS. 6B and 6C are schematic diagrams of a case in whichthe first diffraction element 50 and the second diffraction element 70are not in the conjugated relationship. FIGS. 7A and 7B are schematicdiagrams illustrating a tolerances for a deviation from the conjugatedrelationship between the first diffraction element 50 and the seconddiffraction element 70 illustrated in FIGS. 6B and 6C.

In FIGS. 7A and 7B, light having a specific wavelength is indicated by asolid line Le, light having a specific wavelength of −10 nm is indicatedby a dot-and-dash line Lf, and light having a specific wavelength of +10nm is indicated by a two-dot chain line Lg. Note that, in FIGS. 6A to 6Cand FIGS. 7A and 7B, the first diffraction element 50 and the seconddiffraction element 70 are illustrated in a perspective view, and thefirst diffraction element 50, the second diffraction element 70, and anoptical unit L60 are indicated by arrows.

As illustrated in FIG. 6A, when the first diffraction element 50 and thesecond diffraction element 70 are in the conjugated relationship,divergent light emitted from a point A (a first position) of the firstdiffraction element 50 is converged by the optical unit L90 (lens)having positive power, and is incident on a point B (a second positioncorresponding to the first position) of the second diffraction element70. Thus, a color aberration due to diffraction occurring at the point Bcan be compensated at the point A.

On the other hand, as illustrated in FIGS. 6B and 6C, when the firstdiffraction element 50 and the second diffraction element 70 are not inthe conjugated relationship, divergent light beams emitted from thepoint A of the first diffraction element 50 are converged by the opticalunit L90 (lens) having positive power at the center. However, thedivergent light beams emitted from the point A cross each other in aposition farther or closer with respect to the point B on the seconddiffraction element 70 and are incident. Thus, the point A and the pointB do not have a one-to-one relationship. Here, since a compensationeffect increases when interference fringes are uniform within a region,the compensation effect decreases when the first diffraction element 50and the second diffraction element 70 are not in the conjugatedrelationship. On the other hand, it is difficult to compensate for theentire projection region of the second diffraction element 70 by thefirst diffraction element 50. Thus, in a case of the aspects illustratedin FIGS. 6B and 6C, sufficient wavelength compensation cannot beperformed, and resolution degradation occurs.

Note that there is an error of about ±0.4 mm from the point B that thelight having the specific wavelength reaches in the light having thewavelength of ±10 nm with respect to the specific wavelength, but adecrease in resolution is not noticeable. As a result of considering anallowable range, as illustrated in FIG. 7A, when the light beams havingthe specific wavelength cross each before the point B on the seconddiffraction element 70 where the light beams having the specificwavelength ideally reach, and are incident within a range of ±0.8 mm, adecrease in resolution is not noticeable. Further, as illustrated inFIG. 7B, when the light beams having the specific wavelength cross eachother in the rear of the point B on the second diffraction element 70where the light beams having the specific wavelength ideally reach, andare incident within a range of ±0.8 mm, a decrease in resolution is notnoticeable. Therefore, even when the first diffraction element 50 andthe second diffraction element 70 are not completely in the conjugatedrelationship, a decrease in resolution is allowable when the firstdiffraction element 50 and the second diffraction element 70 aresubstantially in the conjugated relationship, and the light reacheswithin the range of ±0.8 mm from the ideal point B. In other words, inthe present exemplary embodiment, “the first diffraction element 50 andthe second diffraction element 70 have a conjugated relationship” meansthat an incident position of the light having the specific wavelengthfalls within an error range of ±0.8 mm from an ideal incident point.

FIG. 8 is a light beam diagram of the optical system 10 in the presentexemplary embodiment.

In FIG. 8, each optical unit disposed along an optical axis is indicatedby a thick arrow. Further, a light beam emitted from one pixel of theimage light generating device 31 is indicated by a solid line La, a mainlight beam emitted from an end portion of the image light generatingdevice 31 is indicated by a dot-and-dash line Lb, and a position inwhich the light beam is in a conjugated relationship with the firstdiffraction element 50 is indicated by a long dashed line Lc. Here, an“intermediate image” is a place where the light beams (solid lines La)emitted from one pixel converge, and a “pupil” is a place where the mainlight beams (dot-and-dash line Lb) at each angle of view converge. FIG.8 also illustrates a path of the light emitted from the image lightgenerating device 31. Note that, in FIG. 8, all optical units areillustrated in a perspective view in order to simplify the drawing.

As illustrated in FIG. 8, in the optical system 10 in the presentexemplary embodiment, the first optical unit L10 having positive power,the second optical unit L20 that includes the first diffraction element50 and has positive power, the third optical unit L30 having positivepower, and the fourth optical unit L40 that includes the seconddiffraction element 70 and has positive power are provided along anoptical path of the image light emitted from the image light generatingdevice 31.

A focal length of the first optical unit L10 is L/2. Focal lengths ofthe second optical unit L20, the third optical unit L30, and the fourthoptical unit L40 are all L. Therefore, an optical distance from thesecond optical unit L20 to the third optical unit L30 and an opticaldistance from the third optical unit L30 to the fourth optical unit L40are equal.

In the optical system 10, a first intermediate image P1 of the imagelight is formed between the first optical unit L10 and the third opticalunit L30. A pupil R1 is formed between the second optical unit L20 andthe fourth optical unit L40. A second intermediate image P2 of the imagelight is formed between the third optical unit L30 and the fourthoptical unit L40. The fourth optical unit L40 collimates the image lightand forms an exit pupil R2. At this time, the third optical unit L30causes the image light emitted from the second optical unit L20 to beincident as divergent light on the fourth optical unit L40. The secondoptical unit L20 causes the image light emitted from the first opticalunit L10 to be incident as convergent light on the third optical unitL30. In the optical system 10 in the present exemplary embodiment, thepupil R1 is formed in the vicinity of the third optical unit L30 betweenthe second optical unit L20 and the fourth optical unit L40. Thevicinity of the third optical unit L30 refers to a position, between thesecond optical unit L20 and the third optical unit L30, closer to thethird optical unit L30 than the second optical unit L20, or a position,between the third optical unit L30 and the fourth optical unit L40,closer to the third optical unit L30 than the fourth optical unit L40.

The third optical unit L30 causes, of the image light from one point ofthe image light generating device 31, light having a peripheralwavelength deviated from the specific wavelength deflected by the firstdiffraction element 50 to be incident on a predetermined range of thesecond diffraction element 70. In other words, the first diffractionelement 50 and the second diffraction element 70 are in a conjugatedrelationship or a substantially conjugated relationship. Here, anabsolute value of magnification of the projection on the seconddiffraction element 70 by the third optical unit L30 of the firstdiffraction element 50 ranges from 0.5 times to 10 times. The absolutevalue of the magnification may range from 1 time to 5 times.

Therefore, according to the optical system 10 in the present exemplaryembodiment, the first intermediate image P1 of the image light is formedbetween the projection optical system 32 and the light-guiding opticalsystem 60, the pupil R1 is formed in the vicinity of the light-guidingoptical system 60, the second intermediate image P2 of the image lightis formed between the light-guiding optical system 60 and the seconddiffraction element 70, and the second diffraction element 70 collimatesthe image light and forms the exit pupil R2.

In the optical system 10 in the present exemplary embodiment, the firstintermediate image P1 is formed between the first optical unit L10(projection optical system 32) and the second optical unit L20 (firstdiffraction element 50).

The optical system 10 in the present exemplary embodiment satisfies fourconditions (Condition 1, Condition 2, Condition 3, and Condition 4)described below.

Condition 1: A light beam emitted from one point of the image lightgenerating device 31 forms an image as one point in the retina E0.

Condition 2: An incident pupil of the optical system and a pupil of aneye are conjugated.

Condition 3: The first diffraction element 50 and the second diffractionelement 70 are appropriately disposed so as to compensate for aperipheral wavelength.

Condition 4: The first diffraction element 50 and the second diffractionelement 70 are in a conjugated relationship or a substantiallyconjugated relationship.

More specifically, as clearly seen from the dot-and-dash line Lbillustrated in FIG. 8, a light beam emitted from one point of the imagelight generating device 31 satisfies [Condition 1] that an image isformed as one point in the retina E0. Thus, an observer can visuallyrecognize one pixel. Further, as clearly seen from the solid line Laillustrated in FIG. 8, [Condition 2] that the relationship between theincident pupil of the optical system 10 and the pupil E1 of the eye E isconjugated (conjugation of the pupil) is satisfied. Thus, the entireimage generated by the image light generating device 31 can be visuallyrecognized. Further, [Condition 3] that the first diffraction element 50and the second diffraction element 70 are appropriately disposed so asto compensate for a peripheral wavelength is satisfied. Thus, a coloraberration generated by the second diffraction element 70 can becanceled by performing wavelength compensation. Further, as clearly seenfrom the long dashed line Lc illustrated in FIG. 8, [Condition 4] thatthe first diffraction element 50 and the second diffraction element 70are in a conjugated relationship or a substantially conjugatedrelationship is satisfied. Thus, a light beam can be incident on a placehaving the same interference fringes in the first diffraction element 50and the second diffraction element 70, and wavelength compensation canbe appropriately performed. As described above, degradation ofresolution of an image can be suppressed.

Hereinafter, the image light generating device 31 will be described.

The image light generating device 31 in the present exemplary embodimentemits image light acquired by synthesizing a plurality of color lightbeams from a plurality of image display panels configured to emit imagelight that does not have a polarization characteristic.

FIG. 9 is a perspective view of the image light generating device 31.

As illustrated in FIG. 9, the image light generating device 31 includesa first panel 212R (first light emitting panel), a second panel 212G(second light emitting panel), a third panel 212B (third light emittingpanel), and a cross dichroic prism 213 (color synthesis element). Eachof the first panel, the second panel, and the third panel is a lightemitting panel that does not include a lighting device such as abacklight. Therefore, light that does not have a polarizationcharacteristic is emitted from each of the first panel, the secondpanel, and the third panel.

The first panel 212R includes an image generation region 212 f in whicha plurality of pixels are provided in a matrix, and a non-imagegeneration region. An organic EL element is provided in each of theplurality of pixels. The second panel 212G includes an image generationregion 212 f in which a plurality of pixels are provided in a matrix,and a non-image generation region. An organic EL element is provided ineach of the plurality of pixels. The third panel 212B includes an imagegeneration region 212 f in which a plurality of pixels are provided in amatrix, and a non-image generation region. A top-emitting organic ELelement is provided in each of the plurality of pixels.

In the present exemplary embodiment, the plurality of organic ELelements provided in the image generation region 212 f of the firstpanel 212R emit first image light in a red wavelength region. Further,the plurality of organic EL elements provided in the image generationregion 212 f of the second panel 212G emit second image light in a greenwavelength region. Further, the plurality of organic EL elementsprovided in the image generation region 212 f of the third panel 212Bemit third image light in a blue wavelength region.

Hereinafter, a configuration of the first panel 212R, the second panel212G, and the third panel 212B will be described. The first panel 212R,the second panel 212G, and the third panel 212B differ from each otherin material of a light emitting layer and a transport layer formed of anorganic EL material, but have the same basic configuration of the panel.Therefore, a configuration of the panel will be described below withreference to the first panel 212R.

FIG. 10 is a cross-sectional view illustrating a configuration of oneorganic EL element 35 of the first panel 212R. As illustrated in FIG.10, in the organic EL element 35, a reflective electrode 72, an anode73, a light-emitting functional layer 74, and a cathode 75 are providedon one surface of a substrate 79 in order from the substrate 79 side.The substrate 79 is formed of a semiconductor material such as silicon,for example. The reflective electrode 72 is formed of a light-reflectiveconductive material containing, for example, aluminum, silver, or thelike. More specifically, the reflective electrode 72 may be formed of asingle material such as aluminum, silver, or the like, or may be formedof a layered film of titanium (Ti)/AlCu (aluminum copper alloy), or thelike.

The anode 73 is formed of a conductive material having opticaltransparency, such as indium tin oxide (ITO), for example. Although notillustrated, the light-emitting functional layer 74 is formed of aplurality of layers including a light-emitting layer including anorganic EL material, a hole injecting layer, an electron injectinglayer, and the like. The light-emitting layer is formed of a knownorganic EL material corresponding to each light emission color of red,green, and blue.

The cathode 75 functions as a semi-transmissive reflective layer havingproperties (semi-transmissive reflective properties) that transmit somelight and reflect the remaining light. For example, by forming aphotoreflective conductive material, such as an alloy containing silveror magnesium, into a sufficiently thin film, the cathode 75 having thesemi-transmissive reflective properties can be achieved. The emittedlight from the light-emitting functional layer 74 has a component of aspecific resonance wavelength being selectively amplified duringreciprocation between the reflective electrode 72 and the cathode 75, istransmitted through the cathode 75, and is emitted to an observationside (opposite to the substrate 79). In other words, a plurality oflayers from the reflective electrode 72 to the cathode 75 constitute anoptical resonator 80.

The plurality of layers from the reflective electrode 72 to the cathode75 are covered by a sealing film 76. The sealing film 76 is a film forpreventing entry of air and moisture, and is constituted of a singlelayer or a plurality of layers of an inorganic material or an organicmaterial having optical transparency. A color filter 77 is provided onone surface of the sealing film 76. In the third panel 212B, the colorfilter 77 is constituted of a light-absorbing filter layer that absorbslight in a wavelength range other than the blue wavelength range andtransmits light in the blue wavelength range. Similarly, in the firstpanel 212R, a color filter is constituted of a light-absorbing filterlayer that absorbs light in a wavelength range other than the redwavelength range and transmits light in the red wavelength range. In thesecond panel 212G, a color filter is constituted of a light-absorbingfilter layer that absorbs light in a wavelength range other than thegreen wavelength range and transmits light in the green wavelengthrange.

In the present exemplary embodiment, each of the first panel 212R, thesecond panel 212G, and the third panel 212B includes the opticalresonator 80, and thus light corresponding to each color is emitted byresonance of light at a resonance wavelength. Furthermore, the colorfilter 77 is provided on a light emission side of the optical resonator80, and thus color purity of the light emitted from each of the panels212R, 212G, and 212B is further enhanced.

A cover glass 78 for protecting each of the panels 212R, 212G, and 212Bis provided on one surface of the color filter 77.

As illustrated in FIG. 9, the first panel 212R emits the first imagelight LR in the red wavelength region. Therefore, the image lightemitted from the first panel 212R is incident on the cross dichroicprism 213 as the first image light LR in red. The second panel 212Gemits the second image light LG in the green wavelength region.Therefore, the image light emitted from the second panel 212G isincident on the cross dichroic prism 213 as the second image light LG ingreen. The third panel 212B emits the third image light LB in the bluewavelength region. Therefore, the image light emitted from the thirdpanel 212B is incident on the cross dichroic prism 213 as the thirdimage light LB in blue.

A peak wavelength in the red wavelength region is, for example, greaterthan or equal to 630 nm and less than or equal to 680 nm. A peakwavelength in the green wavelength region is, for example, greater thanequal to 495 nm and less than or equal to 570 nm. A peak wavelength inthe blue wavelength region is, for example, greater than or equal to 450nm and less than or equal to 490 nm. Each of the first image light LR,the second image light LG, and the third image light LB is light thatdoes not have a polarization characteristic. In other words, each of thefirst image light LR, the second image light LG, and the third imagelight LB is unpolarized light that does not have a specific vibrationdirection. Note that unpolarized light, namely, light that does not havea polarization characteristic is light that is not in a completelyunpolarized state and includes a polarization component to some extent.For example, the light has a degree of polarization to the extent thatdoes not actively affect an optical member such as a dichroic film, forexample, in terms of optical performance, for example, a degree ofpolarization of less than or equal to 20%.

The cross dichroic prism 213 is constituted of a transparent memberhaving a quadrangular columnar shape. The cross dichroic prism 213includes a first incident surface 213 a, a third incident surface 213 cfacing the first incident surface 213 a, a second incident surface 213 bcontacting perpendicularly to the first incident surface 213 a and thethird incident surface 213 c, an emission surface 213 d facing thesecond incident surface 213 b, a fifth surface 213 e contactingperpendicularly to the first incident surface 213 a, the second incidentsurface 213 b, the third incident surface 213 c, and the emissionsurface 213 d, and a sixth surface 213 f facing the fifth surface 213 e.

The cross dichroic prism 213 includes a first dichroic film DM1 thatdoes not have a polarization separation characteristic, and a seconddichroic film DM2 that does not have a polarization separationcharacteristic. The first dichroic film DM1 and the second dichroic filmDM2 cross each other at an angle of 90°. Hereinafter, an axis in whichthe first dichroic film DM1 and the second dichroic film DM2 cross eachother is referred to as a cross axis CR.

The “dichroic film that does not have a polarization separationcharacteristic” in the specification is a film having a substantiallysimilar wavelength separation characteristic regardless of apolarization direction (S-polarized light, P-polarized light) of lightincident on the dichroic film. A specific example is indicated below todefine the “dichroic film that does not have a polarization separationcharacteristic”.

FIG. 11 is a diagram illustrating one example of atransmittance-wavelength characteristic of a dichroic film that does nothave a polarization separation characteristic.

In FIG. 11, the horizontal axis is a wavelength [nm], and the verticalaxis is a transmittance [%]. A solid line graph indicated by a referencenumeral Ts indicates a transmittance-wavelength characteristic ofS-polarized light, and a dashed line graph indicated by a referencenumeral Tp indicates a transmittance-wavelength characteristic ofP-polarized light.

The “dichroic film that does not have a polarization separationcharacteristic” refers to a film having, when unpolarized light isincident on the dichroic film, a similar trend in the transmittance Tsof the S-polarized light and the transmittance Tp of the P-polarizedlight in a wavelength region to be controlled, such as a wavelengthregion A in FIG. 11: for example, a blue wavelength region of greaterthan or equal to 450 nm and less than or equal to 490 nm, a wavelengthregion B: for example, a green wavelength region of greater than orequal to 495 nm and less than or equal to 570 nm, and a wavelengthregion C: for example, a red wavelength region of greater than or equalto 630 nm and less than or equal to 680 nm. It also refers to the filmhaving a characteristic such that an average difference between thetransmittance Ts and the transmittance Tp in each of the wavelengthregions is less than or equal to 30% and may be less than or equal to10%.

Note that a transmittance is used for expression in FIG. 11, but thesame is true when a reflectance is used for expression. The “dichroicfilm that does not have a polarization separation characteristic” refersto a film having, when unpolarized light is incident on the dichroicfilm, a similar trend in a reflectance of S-polarized light and areflectance of P-polarized light in a wavelength region to becontrolled, and having a characteristic such that an average differencein reflectance in each wavelength region is less than or equal to 30%and may be less than or equal to 10%.

The first dichroic film DM1 has a characteristic so as to reflect thefirst image light LR and transmit the second image light LG and thethird image light LB. The second dichroic film DM2 has a characteristicso as to reflect the second image light LG and transmit the first imagelight LR and the third image light LB. In this way, the first imagelight LR, the second image light LG, and the third image light LB aresynthesized, and the image light L0 in full color is emitted from theemission surface 213 d.

The first panel 212R is disposed so as to face the first incidentsurface 213 a. The second panel 212G is disposed so as to face thesecond incident surface 213 b. The third panel 212B is disposed so as toface the third incident surface 213 c.

Each of the first panel 212R, the second panel 212G, and the third panel212B includes the rectangular image generation region 212 f having longsides and short sides. The image generation region 212 f is a region ofthe surface facing the cross dichroic prism 213 of each panel except fora non-image generation region of a peripheral portion and thatsubstantially generates an image. Each of the first panel 212R, thesecond panel 212G, and the third panel 212B faces the cross dichroicprism 213, and is disposed such that a longitudinal direction FR of theimage generation region 212 f is parallel to the cross axis CR of thecross dichroic prism 213.

Each of the first panel 212R, the second panel 212G, and the third panel212B may include an external terminal regions for electrically couplinga drive circuit board (not illustrated) and a panel outside the longsides of the image generation region. In this case, the externalterminal region does not contribute to generation of an image, and thusmay not need to be disposed so as to face the cross dichroic prism 213.Therefore, each of the first panel 212R, the second panel 212G, and thethird panel 212B may be bonded together such that the image generationregion faces the cross dichroic prism 213 and the external terminalregion protrudes to the outside of the cross dichroic prism 213.

A direction in which the external terminal region of each panelprotrudes to the outside of the cross dichroic prism 213 is notparticularly limited as long as the direction is not a direction inwhich adjacent panels do not interfere with each other. For example, thefirst panel 212R may protrude to the left side of the cross dichroicprism 213 in FIG. 9, the second panel 212G may protrude to the upperside of the cross dichroic prism 213 in FIG. 9, and the third panel 212Bmay protrude to the right side of the cross dichroic prism 213 in FIG.9. Alternatively, the first panel 212R and the third panel 212B may bothprotrude to the right side of the cross dichroic prism 213 in FIG. 9,and the second panel 212G may protrude to the lower side of the crossdichroic prism 213 in FIG. 9.

In the present exemplary embodiment, as described above, each of thefirst panel 212R, the second panel 212G, and the third panel 212B isdisposed such that the longitudinal direction FR of the image generationregion is parallel to the cross axis CR of the cross dichroic prism 213.Thus, a dimension A of one side of the cross dichroic prism 213 can bemade smaller than that when the longitudinal direction FR of the imagegeneration region 212 f is disposed perpendicular to the cross axis CRof the cross dichroic prism 213.

According to the image display device 100 in the present exemplaryembodiment, the first diffraction element 50 and the second diffractionelement 70 formed of the reflection-type volume holograms 85 and 86 areused, and the first diffraction element 50 and the second diffractionelement 70 are appropriately disposed. Thus, a color aberration can becorrected, and degradation of resolution of an image due to the coloraberration can be suppressed.

However, according to consideration of the inventors, a wavelength widthof a reflection wavelength region in a reflection-type hologram isextremely narrow, for example, 20 nm. Thus, in a known display deviceusing two reflection-type holograms, a lot of image light emitted fromthe display is not reflected by the holograms and is transmitted throughthe holograms. As a result, it was found that there is a problem in thatlight-guiding efficiency from the display to an eye of an observerdecreases to, for example, less than or equal to 10%.

For this problem, in the image display device 100 in the presentexemplary embodiment, the above-described problem is solved by acombination of the following configurations.

In a case of the image display device 100 according to the presentembodiment, the first panel 212R, the second panel 212G, and the thirdpanel 212B are constituted of a light emitting panel in which each pixelincludes the organic EL element 35. Therefore, light that does not havea polarization characteristic is emitted from each of the first panel212R, the second panel 212G, and the third panel 212B. Further, theimage light emitted from the first panel 212R, the second panel 212G,and the third panel 212B including the organic EL elements has highcontrast and is bright.

A general cross dichroic prism has two dichroic films having apolarization characteristic. It is assumed that a display deviceincluding this cross dichroic prism is a display device in a comparativeexample. In the display device in the comparative example, in a case inwhich image light is unpolarized light, that is, light that does nothave a polarization characteristic, only one linear polarized light beamamong two linear polarized light beams that are included in the imagelight and are orthogonal to each other contributes to formation of animage, and the other linear polarized light beam does not contribute tothe formation of the image. Thus, in the display device in thecomparative example, the light utilization efficiency is low and abright image is not acquired.

In contrast, in the image display device 100 in the present exemplaryembodiment, the cross dichroic prism 213 includes the first dichroicfilm DM1 and the second dichroic film DM2 that do not have apolarization separation characteristic. In this way, even though theimage light L0 is light that does not have a polarizationcharacteristic, both linear polarized light beams can contribute toformation of an image. Thus, according to the image display device 100in the present exemplary embodiment, the light utilization efficiency ishigher and a brighter image is acquired as compared to the displaydevice in the comparative example.

Further, in the image display device 100 in the present exemplaryembodiment, a bright image is acquired, and thus there is no need tounnecessarily increase a current supplied to the first panel 212R, thesecond panel 212G, and the third panel 212B. Thus, a life of the organicEL element can be maintained, and rapid degradation of the brightnesscan be suppressed.

Further, in the image display device 100 in the present exemplaryembodiment, the image light L0 is guided in air between the firstdiffraction element 50 and the second diffraction element 70, and amember such as a light-guiding plate is not used. Thus, the weight of afront portion of the image display device 100 can be reduced, and a loadapplied to a nose can be reduced. In this way, the image display device100 is less likely to slip off, and the comfort of the image displaydevice 100 can be improved.

Further, in the image light generating device 31, as described above,each of the first panel 212R, the second panel 212G, and the third panel212B is disposed such that the longitudinal direction FR of the imagegeneration region is parallel to the cross axis CR of the cross dichroicprism 213. Thus, a dimension A of one side of a surface of the crossdichroic prism 213 perpendicular to the cross axis CR can be reduced. Asa result, size reduction of the image display device 100 can beachieved.

Note that the technical scope of the present disclosure is not limitedto the above-described exemplary embodiment, and various modificationscan be made to the above-described exemplary embodiment withoutdeparting from the spirit and gist of the present disclosure.

For example, in the exemplary embodiments described above, each of thefirst panel 212R, the second panel 212G, and the third panel 212B isconstituted of an organic EL panel including a pixel having an organicEL element, but may also be constituted of an inorganic light-emittingdiode (LED) panel including a pixel having an inorganic LED element.

Further, in the exemplary embodiment described above, an example hasbeen illustrated in which the first diffraction element and the seconddiffraction element are constituted of a reflection-type volumehologram, but the first diffraction element and the second diffractionelement may be constituted of another hologram element, for example, asurface relief hologram, a blazed hologram, and the like. Even whenthese hologram elements are used, a thin diffraction element having highdiffraction efficiency is acquired.

Further, the specific configuration of the image display deviceexemplified in the exemplary embodiment described above such as thenumber, arrangement, shape, and the like of each component may beappropriately changed.

What is claimed is:
 1. An image display device, comprising: an image light generating device; a first optical unit having positive power; a second optical unit including a first diffraction element and having positive power; a third optical unit having positive power; and a fourth optical unit including a second diffraction element and having positive power, the first to fourth optical units being provided along an optical path of image light emitted from the image light generating device, wherein, on the optical path, a first intermediate image of the image light is formed between the first optical unit and the third optical unit, a pupil is formed between the second optical unit and the fourth optical unit, a second intermediate image of the image light is formed between the third optical unit and the fourth optical unit, an exit pupil is formed at an opposite side of the fourth optical unit from the third optical unit, the image light generating device includes a first light emitting panel configured to emit first image light in a red wavelength region, a second light emitting panel configured to emit second image light in a green wavelength region, a third light emitting panel configured to emit third image light in a blue wavelength region, and a color synthesis element configured to synthesize the first image light, the second image light, and the third image light, the color synthesis element is constituted of a cross dichroic prism including a first dichroic film and a second dichroic film that intersect with each other, and each of the first dichroic film and the second dichroic film does not have a polarization separation characteristic.
 2. The image display device according to claim 1, wherein each of the first light emitting panel, the second light emitting panel, and the third light emitting panel faces a light incident surface of the cross dichroic prism, and is disposed such that a longitudinal direction of an image generation region is parallel to a cross axis of the first dichroic film and the second dichroic film.
 3. The image display device according to claim 1, wherein each of the first light emitting panel, the second light emitting panel, and the third light emitting panel includes a pixel including an organic electroluminescence element.
 4. The image display device according to claim 3, wherein the organic electroluminescence element includes an optical resonator.
 5. The image display device according to claim 1, wherein each of the first light emitting panel, the second light emitting panel, and the third light emitting panel includes a pixel including an inorganic light-emitting diode element.
 6. The image display device according to claim 1, wherein the first diffraction element and the second diffraction element each are constituted of a reflection-type volume hologram. 